In recent years, there has been a rapid increase in demand for delivery of real-time applications and services in computer networks, including Pseudo-Wire Emulation (PWE), Voice over IP (VoIP), video conferencing, and broadcast, multicast and manycast streaming services such as H.261, H.323, and IPTV. These real-time services typically require highly accurate timing to ensure high service quality. Highly accurate timing is also required at base stations in wireless networks based on technologies such as Global System for Mobile communications (GSM), Code Division Multiple Access (CDMA), WiMAX, and Long Term Evolution (LTE).
To ensure high service quality and to facilitate network management, it is desirable to eliminate clock mismatch between computer network equipment such as switches, routers, and base stations. This can be done by providing a highly accurate timing reference at each node, such as a Global Positioning System (GPS) reference or a lower quality oscillator such as a Stratum 2 rubidium oscillator, where the specification for Stratum 2 clock quality is given in Telcordia GR-1244-CORE. However, at the same time it is desirable to reduce the substantial cost resulting from per-node deployment of these timing references.
To reduce per-node cost, it is desirable to use a cheaper oscillator, such as an oven controlled crystal oscillator (OCXO) or a temperature controlled oscillator (TCXO), at each client node. However, OCXO's and TCXO's may be unable to meet Stratum 2 clock quality requirements, or the corresponding clock quality requirements for wireless networks defined in ITU-T G.823 SEC, 3GPP, and IEEE 802.16e. To solve this problem, one or more clock servers may provide timing through a data network to each client node. Each clock server may obtain timing derived from a Stratum 1 reference, such as the Global Positioning System (GPS) or a primary reference source locked to GPS.
One of the important factors that limits the accuracy of timing distribution in packet networks is variations in network delay, known as jitter or packet delay variation (PDV), experienced by timing packets sent between a clock server and client nodes. A primary source of jitter is the multiplexing of timing packets with other packet traffic. The timing accuracy of the timing distribution protocol Network Time Protocol version 4 (NTPv4) over the public Internet may be on the order of 10 milliseconds; in local area networks, the timing accuracy of NTPv4 may be better, on the order of hundreds of microseconds. However, the timing accuracy of conventional NTPv4 appears to be insufficient for distribution of Stratum 2 quality timing.
One approach to enhancing the accuracy of timing distribution in packet networks is to use special approaches to minimize the jitter affecting timing packets sent between the clock server and the client nodes. To meet the above-described clock quality requirements, these special approaches typically must go beyond simply assigning timing packets to a highest priority class of service, such as Expedited Forwarding (EF), as defined by the Internet Engineering Task Force (IETF) Differentiated Services Working Group. An example is controlled delay packet forwarding, described in Barry et al., “Apparatus and Method of Controlled Delay Packet Forwarding,” U.S. Ser. No. 11/843,493, filed on Aug. 22, 2007. However, to meet the above-described clock quality requirements, each switch and/or router traversed by timing packets often must support one or more of these approaches.
It is therefore desirable to determine how to enhance the accuracy of timing distribution in packet networks without imposing special requirements on each switch and/or router traversed by timing packets sent between a clock server and client nodes. In particular, it is desirable to determine how to achieve this enhancement in accuracy via processing at the client nodes of delay values associated with the timing packets.